1. Field of the Invention
This invention relates to a synthetic lubricating oil and, more specifically, it relates to a synthetic lubricating oil which comprises a specific carbonic acid ester as a major component (a base oil) and is especially useful as a lubricating oil for refrigerators and the like (the oil being hereinafter referred to as "a refrigerator oil").
2. Prior Art
Generally, naphthenic mineral oils, paraffinic mineral oils, alkylbenzenes, polyglycolic oils and mixtures thereof, which have each a kinematic viscosity of 10-200 cSt at 40.degree. C., as well as said oils incorporated with suitable additives, have been used as refrigerator oils.
On the other hand, chlorofluorocarbon type refrigerants (CFCS), such as CFC-11, CFC-12, CFC-115 and HCFC-22, have been used as such for refrigerators.
Of these CFCS, those such as CFC-11, CFC-12 and CFC-115, which are obtained by substituting all the hydrogen atoms of their hydrocarbons by halogen atoms including chlorine atoms, may lead to the destruction of the ozone layer, and, therefore, the use of the CFCS has been restricted. Accordingly, hydrogen-containing halogenocarbons, particularly chlorine-free type halogenocarbons such as HFC-134a and HFC-152a, have been being used as substitutes for CFCS. HFC-134a is especially hopeful as a substitute refrigerant since it is similar in thermodynamic properties to CFC-12 which has heretofore been used in many kinds of refrigerators for home cold-storage chests, air-conditioners, car air-conditioners and the like.
Refrigerator oils require various properties, among which their miscibility with refrigerants is extremely important in regard to lubricity and system efficiency in refrigerators. However, conventional refrigerator oils comprising, as the base oils, naphthenic oils, paraffinic oils, alkylbenzenes and the like, are hardly miscible with chlorine-free type halogenocarbons such as HFC-134a. Therefore, if said conventional refrigerator oils are used in combination with HFC-134a, the resulting mixture will separate into two layers at normal temperature thereby to degrade the oil-returnability which is the most important within the refrigeration system and cause various troubles such as a decrease in refrigeration efficiency, the deterioration of lubricity and the consequent seizure of the compressor within the system whereby the refrigerator oils are made unsuitable for use as such.
In attempts to solve such problems, the present inventors developed polyglycolic refrigerator oils which have excellent miscibility with HFC-134a as compared with conventional known refrigerator oils, filed an application for a patent for the thus developed polyglycolic refrigerator oils and have already obtained a patent (U.S. Pat. No. 4,948,525) therefor. Further, U.S. Pat. No. 4,755,316 discloses polyglycolic refrigerator oils which are compatible with HFC-134a.
On the other hand, refrigerator oils used in compressors of home refrigerators and the like are required to have a high electrical insulating property. Among the known refrigerator oils, alkylbenzenes and the mineral oils have the highest insulating property, but they are hardly miscible with chlorine-free type halogenocarbons such as HFC-134a as mentioned above. Further, the polyglycolic oils having excellent miscibility with HFC-134a raise problems as to their inferior electrical insulating property.
The present inventors found that esters having specific structures have excellent miscibility with chlorine-free type halogenocarbons such as HFC-134a and a high electrical insulating property, and filed applications for patents for the esters (Japanese Patent Applications Nos. Hei. 1-341244, Hei. 1-341245, Hei. 2-105772 and Hei. 2-121133. Hei. 1 and Hei. 2 are the same as 1989 and 1990 A.D., respectively.).
The known esters have various excellent properties as mentioned above, but they are hydrolyzable and therefore they are not necessarily suitable for use in apparatuses, such as cold-storage chests and car air-conditioners, which must be operated for a long period of time with high reliability. Accordingly, it has been desired to develop a refrigerator oil which has high miscibility with hydrogen-containing halogenocarbons such as HFC-134a, and also has a high electrical insulating property and high hydrolysis stability (or high stability to hydrolysis).
Further, conventional ester type synthetic oils have heretofore been known as synthetic oils having excellent heat resistance, but they are hydrolyzable as mentioned above, and therefore, it has been desired to develop an ester type synthetic oil having excellent hydrolytic stability.